Microwave dewaxing

ABSTRACT

Method and apparatus for dewaxing a shell mold assembly containing a wax-type pattern therein which involves incorporating into the mold structure a material having a high loss factor in the microwave frequency range and then subjecting the mold assembly to microwave energy in the frequency range for a period of time sufficient to liquefy the wax-type pattern material.

tively, the lossy material can be applied to the exterior coat by means of spraying or dipping from a suspension y of the lossy material.

The following specific examples illustrate the results achieved employing different variations of the process:

EXAMPLE l A prime layer of slurry was made up containing 25% byl weight graphite particles, the slurry having the following composition:

30% colloidal silica binder 45% suitably sized zircon flour 25% suitably sized graphite After the mold was completely built up, it was subjected to microwave irradiation at a frequency of 2450 mH vat a power output of about 7.2 kilowatts. lt was found that the mold thus produced was dewaxed in about 3 minutes.y

i EXAMPLE 2 The same type of mold as specified in Example l was b'uilt up, except that the graphite particles were included in an amount of 25 percent by weight in each of 2 prime layers. Under the same conditions ofirradiation as in Example l, the mold was found to be dewaxed in2 minutes.

EXAMPLE 3 EXAMPLE 5 ln this example, the finished mold was dipped in an aqueous suspension of carbon particles in which the carbon constitued 30 percent by weight of the suspension. Dewaxing under the conditions previously mentioned occurred in 2 minutes.

EXAMPLE 6 A finished mold assembly was sprayed with an aqueous suspension containing l5 percent by weight of carbon particles of colloidal size. The mold assembly was dewaxed in approximately 2 minutes under the aforementioned conditions of dewaxing.

EXAMPLE 7 ln this example, the graphite particles were included in one of the prime coats in a percent concentration by weight. The concrete layernext to the prime coat consisted of a fifty-fifty mixture of carbon particles in sand. lt was found that this mold could be dewaxed in about 2 minutes.

The amount of exposure to microwave radiation in order to achieve dewaxing is dependent on at least four factors. These are the configuration of the shell, the

type of lossy additives, the concentration of the additive, and the power level of the microwave source. The use of auxiliary heat following microwave heating was also found to decrease the time of microwave exposure required to about one half. Tests made on production size molds at a power level of l0 kilowatts for 4 to 5 minutes followed by exposure to a hot air oven at 200F. for an equivalent amount of time showed that such molds could be completely freed of wax.

A continuous furnacing setup for microwave dewaxing is shown in the accompanying drawing. In the drawing, reference numeral 10 indicates generally a table on which the various units are supported, including a metal vestibule 11 into which the moldV clusters to be dewaxed are introduced. The clusters are supported on suitable pallets (not shown) which are received on drive rollers 12 and are driven by a drive chain and sprocket or other suitable driving means located in a housing 13. As the pallets are propelled along the rollers 12, they engage a microswitch or other switch means to actuate a pair of air cylinders 14 and thereby lift a metallic baffle 15 which serves as a shield to prevent the high frequency energy from radiating into the vestibule 11. As the pallet proceeds through the furnace, it engages other switch means (not shown) which deenergizes the air cylinders 14 to lower'the baffle 15 into its shielding position and energizes a pair of air cylinders 16 to raise a baffle 17 and thereby permit entry into the microwave treating section. This section is energized by a pair of microwave generators 18 and'l9 which deliver microwave energy through a system of wave guides 20 and 21 at the mold clusters supported on the pallets. The entire system, including the hot air system, trough discharge and the conveyor are under the control of a control unit 22.

After a predetermined residence time in .passing y through the'microwave section, the molds are passed through a hot air zone in which there is located a blower housing 23 containing a blower 3l which di` rects hot air at a temperature of about to 190F. at the mold assembly to remove the last traces of any waxy material contained therein. The movement of the pallet through the hot air zone then actuates another switch (not shown) to energize a pair-of air cylinders 24, thereby raising a metal baffle 25 permitting egress from the hot air zone. When the baffle 25 is raised, a succeeding baffle 26 is still in its lowered or shielding position, and after the pallet passes the baffle 25, it energizes still another switch means to deenergize the air cylinders 24 and energizes a-pair of air cylinders 27 to raise the baffle 26 and thereby permits the dewaxed mold assembly to be withdrawn from the conveyor at an exit vestibule 28.

The pallets are suitably apertured so that the molten wax drops down into a trough 29 which extends the length of the conveyor from the beginning of the microwave zone to the exit vestibule. A discharge 30 is provided to deliver the recovered waxy material to filtering or other purifying means so that it may be reused in the manufacture of patterns.

4 The microwave method of pattern removal involves v the rapid generation of heat at the optimum place required, at or near the pattern or mold material because of the penetrating power of high frequency radiation. Heat transfer within the mold is therefore by conduction rather than by a slower combination of convection and/or thermal radiation plus conduction, as in other 

1. The method of dewaxing a shell mold containing a wax-type pattern therein which comprises incorporating in the mold structure a material having a high loss factor in the frequency range of 300 to 30,000 megahertz and subjecting said mold to radiating microwave energy in said frequency range for a period of time sufficient to liquefy the wax pattern.
 2. The method of claim 1 in which said loss factor is at least 1.0.
 3. The method of claim 1 in which said frequency range extends from 900 to 2500 megahertz.
 4. The method of claim 1 in which said material is graphite.
 5. The method of claim 1 in which said material is carbon.
 6. The method of claim 4 in which said graphite is applied in a coating forming the exterior coating of said mold.
 7. The method of claim 1 in which the mold, after being subjected to said microwave energy is contacted with a stream of hot air.
 8. An apparatus for dewaxing a shell mold comprising, in sequence, an inlet zone, a heating zone containing a source of radiating microwave energy, and an exit zone, conveyor means extending through said inlet zone, said heating zone and said exit zone, means for delivering a shell mold sequentially and continuously through said zones, and collector means for collecting molten wax melted out of said shell mold in said heating zone.
 9. The apparatus of claim 8 which includes a second heating zone including hot air circulating means between said heating zone and said exit zone.
 10. The apparatus of claim 8 which inCludes reflector means selectively positionable between said inlet zone and said heating zone to shield said inlet zone from radiation from said heating zone. 